I will assume that Curtis charge $3.70 per mile
so
Let
y -----> the total charge
x -----> the number of miles
we have that
Audrey's equation
y=ax+4
where a is the amount that Audrey charge per mile
Curtis equation
y=3,70x
we know that
For a distance of 30 miles the charge is the same in both equations
so
ax+4=3.70x
For x=30
30a+4=3.70(30)
Solve for a
30a+4=111
30a=111-4
30a=107
a=3.57
that means -----> Audrey charge $3.57 per mile